Fie. i. 



Source 



Pi-.f. A. Gly, Mr. V. .). lilyth, an.l Mr. -I. S. I Mini,,],. 



so produced a rhan^e in the number of interference bands, which 

 measured the retardation of one ray relatively to the other. The 

 elongation was thus measured by the number of interference bands 

 which passed across the field of view of the observing apparatus. The 



weight being kept hanging on the 

 wire, the temperature was alter- 

 - nately raised and lowered a num- 

 ber of times until the number of 

 bands measuring the change be- 

 came approximately constant. 



It was noticed that the first 

 heating produced a diminution in 

 the number of bands measuring 

 the elongation, and the succeeding 

 heatings an increase which finally 

 settled down to a constant value. 

 In our experiments the method 

 adopted was entirely different. 

 The specimens were wires of radius 

 varying from about - 036 cm. to 

 0'06 cm., and each had a length of 

 about 5 metres. The exact dimen- 

 sions are given in the tables of 

 results below. The experimental 

 arrangement followed in the case 

 of each wire was practically the 

 same. 



The wire was hung vertically in 

 the laboratory from a plate at the 

 top screwed firmly to the lower side 

 of a wooden beam in the ceiling. 

 The upper end was secured by 

 being passed through a hole in the 

 plate and firmly soldered at the 

 back. The wire passed along the 

 axis of a double-walled tube or 

 jacket (fig. 1), consisting of two 

 coaxial cylinders of tin-plate of 

 diameters 3'5 cm. and 5 cm. re- 

 spectively. The upper and lower 



ends of the jacket were closed by corks, through which the wire 

 loosely passed. A scale-pan weighing about 4 Ibs. was hung on the 

 lower end of the wire to receive weights for the stretching experi- 

 ments, and served when no other weight was applied to keep the wire 

 taut. 



Sink 



